Early Forebrain Neurons and Scaffold Fibers in Human Embryos.

Neural progenitor proliferation, neuronal migration, areal organization, and pioneer axon wiring are critical events during early forebrain development, yet remain incompletely understood, especially in human. Here, we studied forebrain development in human embryos aged 5 to 8 postconceptional weeks (WPC5-8), stages that correspond to the neuroepithelium/early marginal zone (WPC5), telencephalic preplate (WPC6 & 7), and incipient cortical plate (WPC8). We show that early telencephalic neurons are formed at the neuroepithelial stage; the most precocious ones originate from local telencephalic neuroepithelium and possibly from the olfactory placode. At the preplate stage, forebrain organization is quite similar in human and mouse in terms of areal organization and of differentiation of Cajal-Retzius cells, pioneer neurons, and axons. Like in mice, axons from pioneer neurons in prethalamus, ventral telencephalon, and cortical preplate cross the diencephalon-telencephalon junction and the pallial-subpallial boundary, forming scaffolds that could guide thalamic and cortical axons at later stages. In accord with this model, at the early cortical plate stage, corticofugal axons run in ventral telencephalon in close contact with scaffold neurons, which express CELSR3 and FZD3, two molecules that regulates formation of similar scaffolds in mice.

The evolution of cortical development: the synapsid-diapsid divergence.

The cerebral cortex covers the rostral part of the brain and, in higher mammals and particularly humans, plays a key role in cognition and consciousness. It is populated with neuronal cell bodies distributed in radially organized layers. Understanding the common and lineage-specific molecular mechanisms that orchestrate cortical development and evolution are key issues in neurobiology. During evolution, the cortex appeared in stem amniotes and evolved divergently in two main branches of the phylogenetic tree: the synapsids (which led to present day mammals) and the diapsids (reptiles and birds). Comparative studies in organisms that belong to those two branches have identified some common principles of cortical development and organization that are possibly inherited from stem amniotes and regulated by similar molecular mechanisms. These comparisons have also highlighted certain essential features of mammalian cortices that are absent or different in diapsids and that probably evolved after the synapsid-diapsid divergence. Chief among these is the size and multi-laminar organization of the mammalian cortex, and the propensity to increase its area by folding. Here, I review recent data on cortical neurogenesis, neuronal migration and cortical layer formation and folding in this evolutionary perspective, and highlight important unanswered questions for future investigation.

Seven pass Cadherins CELSR1-3.

Cadherin EGF LAG seven-pass G-type receptors 1, 2 and 3 (CELSR1-3) form a family of three atypical cadherins with multiple functions in epithelia and in the nervous system. During the past decade, evidence has accumulated for a key role of CELSR1 in epithelial planar cell polarity (PCP), and for CELSR2 and CELSR3 in ciliogenesis and neural development, especially neuron migration and axon guidance in the central, peripheral and enteric nervous systems. Phenotypes in mutant mice indicate that CELSR proteins work in concert with FZD3 and FZD6, but several questions remain. Apart from PCP signaling pathways implicating CELSR1 that begin to be unraveled, little is known about other signals generated by CELSR2 and CELSR3. A crucial question concerns the putative ligands that trigger signaling, in particular what is the role of WNT factors. Another critical issue is the identification of novel intracellular pathways and effectors that relay and transmit signals in receptive cells? Answers to those questions should further our understanding of the role of those important molecules not only in development but also in regeneration and disease.

Shaping the nervous system: role of the core planar cell polarity genes.

Planar cell polarity (PCP) is complementary to the intrinsic polarization of single cells and refers to the global coordination of cell behaviour in the plane of a tissue and, by extension, to the signalling pathways that control it. PCP is most evident in cell sheets, and research into PCP was for years confined to studies in Drosophila melanogaster. However, PCP has more recently emerged as an important phenomenon in vertebrates, in which it regulates various developmental processes and is associated with multiple disorders. In particular, core PCP genes are crucial for the development and function of the nervous system. They are involved in neural tube closure, ependymal polarity, neuronal migration, dendritic growth and axon guidance.

Patterning of papillae on the mouse tongue: A system for the quantitative assessment of planar cell polarity signaling.

The dorsal surface of the mouse tongue is covered by ~7000 papillae, asymmetric epithelial protrusions that are precisely oriented to create a stereotyped macroscopic pattern. Within the context of this large-scale pattern, neighboring papillae exhibit a high degree of local order that minimizes the differences in their orientations. We show here that the orientations of lingual papillae are under the control of the core planar cell polarity (PCP) genes Vangl1, Vangl2, and Celsr1. Using K14-Cre and Nkx2.5-Cre to induce conditional knockout of Vangl1 and/or Vangl2 in the tongue epithelium, we observe more severe disruptions to local order among papillae with inactivation of larger numbers of Vangl genes, a greater role for Vangl2 than Vangl1, and a more severe phenotype with the Vangl2 Looptail (Lp) allele than the Vangl2 null allele, consistent with a dominant negative mode of action of the Vangl2Lp allele. Interestingly, Celsr1-/- tongues show disruption of both local and global order, with many papillae in the anterior tongue showing a reversed orientation. To quantify each of these phenotypes, we have developed and applied three procedures for sampling the orientations of papillae and assessing the degree of order on different spatial scales. The experiments reported here establish the dorsal surface of the mouse tongue as a favorable system for studying PCP control of epithelial patterning.

The atypical cadherin Celsr1 functions non-cell autonomously to block rostral migration of facial branchiomotor neurons in mice.

The caudal migration of facial branchiomotor (FBM) neurons from rhombomere (r) 4 to r6 in the hindbrain is an excellent model to study neuronal migration mechanisms. Although several Wnt/Planar Cell Polarity (PCP) components are required for FBM neuron migration, only Celsr1, an atypical cadherin, regulates the direction of migration in mice. In Celsr1 mutants, a subset of FBM neurons migrates rostrally instead of caudally. Interestingly, Celsr1 is not expressed in the migrating FBM neurons, but rather in the adjacent floor plate and adjoining ventricular zone. To evaluate the contribution of different expression domains to neuronal migration, we conditionally inactivated Celsr1 in specific cell types. Intriguingly, inactivation of Celsr1 in the ventricular zone of r3-r5, but not in the floor plate, leads to rostral migration of FBM neurons, greatly resembling the migration defect of Celsr1 mutants. Dye fill experiments indicate that the rostrally-migrated FBM neurons in Celsr1 mutants originate from the anterior margin of r4. These data suggest strongly that Celsr1 ensures that FBM neurons migrate caudally by suppressing molecular cues in the rostral hindbrain that can attract FBM neurons.

Celsr1 is required for the generation of polarity at multiple levels of the mouse oviduct.

The oviduct is an important organ in reproduction where fertilization occurs, and through which the fertilized eggs are carried to the uterus in mammals. This organ is highly polarized, where the epithelium forms longitudinal folds along the ovary-uterus axis, and the epithelial multicilia beat towards the uterus to transport the ovulated ova. Here, we analyzed the postnatal development of mouse oviduct and report that multilevel polarities of the oviduct are regulated by a planar cell polarity (PCP) gene, Celsr1. In the epithelium, Celsr1 is concentrated in the specific cellular boundaries perpendicular to the ovary-uterus axis from postnatal day 2. We found a new feature of cellular polarity in the oviduct - the apical surface of epithelial cells is elongated along the ovary-uterus axis. In Celsr1-deficient mice, the ciliary motion is not orchestrated along the ovary-uterus axis and the transport ability of beating cilia is impaired. Epithelial cells show less elongation and randomized orientation, and epithelial folds show randomized directionality and ectopic branches in the mutant. Our mosaic analysis suggests that the geometry of epithelial cells is primarily regulated by Celsr1 and as a consequence the epithelial folds are aligned. Taken together, we reveal the characteristics of the multilevel polarity formation processes in the mouse oviduct epithelium and suggest a novel function of the PCP pathway for proper tissue morphogenesis.

Celsr3 and Fzd3 Organize a Pioneer Neuron Scaffold to Steer Growing Thalamocortical Axons.

Celsr3 and Fzd3 regulate the development of reciprocal thalamocortical projections independently of their expression in cortical or thalamic neurons. To understand this cell non autonomous mechanism further, we tested whether Celsr3 and Fzd3 could act via Isl1-positive guidepost cells. Isl1-positive cells appear in the forebrain at embryonic day (E) 9.5-E10.5 and, from E12.5, they form 2 contingents in ventral telencephalon and prethalamus. In control mice, corticothalamic axons run in the ventral telencephalic corridor in close contact with Isl1-positive cells. When Celsr3 or Fzd3 is inactivated in Isl1-expressing cells, corticofugal fibers stall and loop in the ventral telencephalic corridor of high Isl1 expression, and thalamic axons fail to cross the diencephalon-telencephalon junction (DTJ). At E12.5, before thalamic and cortical axons emerge, pioneer projections from Isl1-positive cells cross the DTJ from both sides in control but not mutant embryos. These early projections appear to act like a bridge to guide later growing thalamic axons through the DTJ. Our data suggest that Celsr3 and Fzd3 orchestrate the formation of a scaffold of pioneer neurons and their axons. This scaffold extends from prethalamus to ventral telencephalon and subcortex, and steers reciprocal corticothalamic fibers.

A dual role for planar cell polarity genes in ciliated cells.

In the nervous system, cilia dysfunction perturbs the circulation of the cerebrospinal fluid, thus affecting neurogenesis and brain homeostasis. A role for planar cell polarity (PCP) signaling in the orientation of cilia (rotational polarity) and ciliogenesis is established. However, whether and how PCP regulates cilia positioning in the apical domain (translational polarity) in radial progenitors and ependymal cells remain unclear. By analysis of a large panel of mutant mice, we show that two PCP signals are operating in ciliated cells. The first signal, controlled by cadherin, EGF-like, laminin G-like, seven-pass, G-type receptor (Celsr) 2, Celsr3, Frizzled3 (Fzd3) and Van Gogh like2 (Vangl2) organizes multicilia in individual cells (single-cell polarity), whereas the second signal, governed by Celsr1, Fzd3, and Vangl2, coordinates polarity between cells in both radial progenitors and ependymal cells (tissue polarity). Loss of either of these signals is associated with specific defects in the cytoskeleton. Our data reveal unreported functions of PCP and provide an integrated view of planar polarization of the brain ciliated cells.

Genetic evidence that Celsr3 and Celsr2, together with Fzd3, regulate forebrain wiring in a Vangl-independent manner.

Celsr3 and Fzd3, members of "core planar cell polarity" (PCP) genes, were shown previously to control forebrain axon guidance and wiring by acting in axons and/or guidepost cells. Here, we show that Celsr2 acts redundantly with Celsr3, and that their combined mutation mimics that of Fzd3. The phenotypes generated upon inactivation of Fzd3 in different forebrain compartments are similar to those in conditional Celsr2-3 mutants, indicating that Fzd3 and Celsr2-3 act in the same population of cells. Inactivation of Celsr2-3 or Fzd3 in thalamus does not affect forebrain wiring, and joint inactivation in cortex and thalamus adds little to cortical inactivation alone in terms of thalamocortical projections. On the other hand, joint inactivation perturbs strongly the formation of the barrel field, which is unaffected upon single cortical or thalamic inactivation, indicating a role for interactions between thalamic axons and cortical neurons in cortical arealization. Unexpectedly, forebrain wiring is normal in mice defective in Vangl1 and Vangl2, showing that, contrary to epithelial PCP, axon guidance can be Vangl independent in some contexts. Our results suggest that Celsr2-3 and Fzd3 regulate axonal navigation in the forebrain by using mechanisms different from classical epithelial PCP, and require interacting partners other than Vangl1-2 that remain to be identified.

Antagonistic functions of Dishevelleds regulate Frizzled3 endocytosis via filopodia tips in Wnt-mediated growth cone guidance.

How growth cones detect small concentration differences of guidance cues for correct steering remains a long-standing puzzle. Commissural axons engage planar cell polarity (PCP) signaling components to turn anteriorly in a Wnt gradient after midline crossing. We found here that Frizzled3, a Wnt receptor, undergoes endocytosis via filopodia tips. Wnt5a increases Frizzled3 endocytosis, which correlates with filopodia elongation. We discovered an unexpected antagonism between Dishevelleds, which may function as a signal amplification mechanism in filopodia where PCP signaling is activated: Dishevelled2 blocks Dishevelled1-induced Frizzled3 hyperphosphorylation and membrane accumulation. A key component of apical-basal polarity (A-BP) signaling, aPKC, also inhibits Dishevelled1-induced Frizzled3 hyperphosphorylation. Celsr3, another PCP component, is required in commissural neurons for anterior turning. Frizzled3 hyperphosphorylation is increased in Celsr3 mutant mice, where PCP signaling is impaired, suggesting Frizzled3 hyperphosphorylation does correlate with loss of PCP signaling in vivo. Furthermore, we found that the small GTPase, Arf6, which is required for Frizzled3 endocytosis, is essential for Wnt-promoted outgrowth, highlighting the importance of Frizzled3 recycling in PCP signaling in growth cone guidance. In a Wnt5a gradient, more Frizzled3 endocytosis and activation of atypical protein kinase C was observed on the side of growth cones facing higher Wnt5a concentration, suggesting that spatially controlled Frizzled3 endocytosis is part of the key mechanism for growth cone steering.

A role for atypical cadherin Celsr3 in hippocampal maturation and connectivity.

Atypical cadherin Celsr3, a regulator of planar cell polarity, is critical for the development of the axonal blueprint. We previously showed that expression of Celsr3 is necessary to establish forebrain connections such as the anterior commissure and thalamocortical and corticospinal tracts. The requirement for Celsr3 during hippocampal wiring and its action in the hippocampus remain largely unexplored. Here, we compared the connectivity and maturation of the hippocampal formation in Celsr3|Foxg1 and Celsr3|Dlx mice. Celsr3 is inactivated in the whole telencephalon, including the hippocampal primordium, in Celsr3|Foxg1 mice, and in the early basal telencephalon, including ganglionic eminences and ventral diencephalon, in Celsr3|Dlx mice. Behavioral tests showed that both mutants were hyperactive and had impaired learning and memory. Abnormal cytoarchitecture of CA1, CA3, and dentate gyrus was found in the Celsr3|Foxg1 mutant, in which afferent and efferent hippocampal pathways, as well as intrinsic connections, were dramatically disrupted. In Celsr3|Dlx mutant mice, hippocampal cytoarchitecture was mildly affected and extrinsic and intrinsic connectivity moderately disturbed. In both mutants, pyramidal neurons in CA1 harbored atrophic dendritic trees, with decreased synapse density and increased proportion of symmetric versus asymmetric synapses, and long-term potentiation was altered. In contrast, mutant hippocampal neurons extended neurites that were normal, even longer than those of control neurons, indicating that anomalies in vivo are secondary to defective connections. Postnatal neurogenesis was preserved and mutant interneurons were able to migrate to the hippocampus. Thus, like in neocortex, Celsr3 is required for hippocampal development, connectivity and function, and for pyramidal cell maturation.

Transcriptional mechanisms of EphA7 gene expression in the developing cerebral cortex.

The patterning of cortical areas is controlled by a combination of intrinsic factors that are expressed in the cortex and external signals such as inputs from the thalamus. EphA7 is a guidance receptor that is involved in key aspects of cortical development and is expressed in gradients within developing cortical areas. Here, we identified a regulatory element of the EphA7 promoter, named pA7, that can recapitulate salient features of the pattern of expression of EphA7, including cortical gradients. Using a pA7-Green fluorescent Protein (GFP) mouse reporter line, we isolated cortical neuron populations displaying different levels of EphA7/GFP expression. Transcriptome analysis of these populations enabled to identify many differentially expressed genes, including 26 transcription factors with putative binding sites in the pA7 element. Among these, Pbx1 was found to bind directly to the EphA7 promoter in the developing cortex. All genes validated further were confirmed to be expressed differentially in the developing cortex, similarly to EphA7. Their expression was unchanged in mutant mice defective for thalamocortical projections, indicating a transcriptional control largely intrinsic to the cortex. Our study identifies a novel repertoire of cortical neuron genes that may act upstream of, or together with EphA7, to control the patterning of cortical areas.

Gene expression analysis of the embryonic subplate.

The subplate layer of the cerebral cortex is comprised of a heterogeneous population of cells and contains some of the earliest-generated neurons. In the embryonic brain, subplate cells contribute to the guidance and areal targeting of thalamocortical axons. At later developmental stages, they are predominantly involved in the maturation and plasticity of the cortical circuitry and the establishment of functional modules. We aimed to further characterize the embryonic murine subplate population by establishing a gene expression profile at embryonic day (E) 15.5 using laser capture microdissection and microarrays. The microarray identified over 300 transcripts with higher expression in the subplate compared with the cortical plate at this stage. Using quantitative reverse transcription-polymerase chain reaction, in situ hybridization (ISH), and immunohistochemistry (IHC), we have confirmed specific expression in the E15.5 subplate for 13 selected genes, which have not been previously associated with this compartment (Abca8a, Cdh10, Cdh18, Csmd3, Gabra5, Kcnt2, Ogfrl1, Pls3, Rcan2, Sv2b, Slc8a2, Unc5c, and Zdhhc2). In the reeler mutant, the expression of the majority of these genes (9 of 13) was shifted in accordance with the altered position of subplate. These genes belong to several functional groups and likely contribute to synapse formation and axonal growth and guidance in subplate cells.

DeltaNp73 regulates neuronal survival in vivo.

Apoptosis occurs widely during brain development, and p73 transcription factors are thought to play essential roles in this process. The p73 transcription factors are present in two forms, the full length TAp73 and the N-terminally truncated DeltaNp73. In cultured sympathetic neurons, overexpression of DeltaNp73 inhibits apoptosis induced by nerve growth factor withdrawal or p53 overexpression. To probe the function of DeltaNp73 in vivo, we generated a null allele and inserted sequences encoding the recombinase Cre and green fluorescent protein (EGFP). We show that DeltaNp73 is heavily expressed in the thalamic eminence (TE) that contributes neurons to ventral forebrain, in vomeronasal neurons, Cajal-Retzius cells (CRc), and choroid plexuses. In DeltaNp73(-/-) mice, cells in preoptic areas, vomeronasal neurons, GnRH-positive cells, and CRc were severely reduced in number, and choroid plexuses were atrophic. This phenotype was enhanced when DeltaNp73-positive cells were ablated by diphtheria toxin expression. However, ablation of cells that express DeltaNp73 and Wnt3a did neither remove all CRc, nor did they abolish Reelin secretion or generate a reeler-like cortical phenotype. Our data emphasize the role of DeltaNp73 in neuronal survival in vivo and in choroid plexus development, the importance of the TE as a source of neurons in ventral forebrain, and the multiple origins of CRc, with redundant production of Reelin.

Maturation of "neocortex isole" in vivo in mice.

How much neocortical development depends on connections remains elusive. Here, we show that Celsr3|Dlx mutant mice have no extrinsic neocortical connections yet survive to postnatal day 20, acquire a basic behavioral repertoire, and display spontaneous hyperactivity, with abnormal light/dark activity cycling. Except for hallmarks related to thalamic input, such as barrels in somatosensory cortex, cortical arealization and laminar maturation proceeded normally. However, the tangential extension of the mature cortex was diminished, with radial thickness less severely affected. Deep layer neurons were reduced in number, and their apical and basal dendritic arbors were blunted, with reduced synapse density. Interneurons reached the cortex, and their density was comparable with wild type. The excitability of mutant pyramidal neurons, measured in vitro in patch-clamp experiments in acute slices, was decreased. However, their firing activity in vivo was quite similar to the wild type, except for the presence of rapid firing exhaustion in some mutant neurons. Local field potential and electrocorticogram showed similar range of oscillations, albeit with higher frequency peaks and reduced left-right synchrony in the mutant. Thus, "protomap" formation, namely cortical lamination and arealization, proceed normally in absence of extrinsic connections, but survival of projection neurons and acquisition of mature morphological and some electrophysiological features depend on the establishment of normal cortical-subcortical relationships.

Wnt/planar cell polarity signaling controls the anterior-posterior organization of monoaminergic axons in the brainstem.

Monoaminergic neurons [serotonergic (5-HT) and dopaminergic (mdDA)] in the brainstem project axons along the anterior-posterior axis. Despite their important physiological functions and implication in disease, the molecular mechanisms that dictate the formation of these projections along the anterior-posterior axis remain unknown. Here we reveal a novel requirement for Wnt/planar cell polarity signaling in the anterior-posterior organization of the monoaminergic system. We find that 5-HT and mdDA axons express the core planar cell polarity components Frizzled3, Celsr3, and Vangl2. In addition, monoaminergic projections show anterior-posterior guidance defects in Frizzled3, Celsr3, and Vangl2 mutant mice. The only known ligands for planar cell polarity signaling are Wnt proteins. In culture, Wnt5a attracts 5-HT but repels mdDA axons, and Wnt7b attracts mdDA axons. However, mdDA axons from Frizzled3 mutant mice are unresponsive to Wnt5a and Wnt7b. Both Wnts are expressed in gradients along the anterior-posterior axis, consistent with their role as directional cues. Finally, Wnt5a mutants show transient anterior-posterior guidance defects in mdDA projections. Furthermore, we observe during development that the cell bodies of migrating descending 5-HT neurons eventually reorient along the direction of their axons. In Frizzled3 mutants, many 5-HT and mdDA neuron cell bodies are oriented abnormally along the direction of their aberrant axon projections. Overall, our data suggest that Wnt/planar cell polarity signaling may be a global anterior-posterior guidance mechanism that controls axonal and cellular organization beyond the spinal cord.

Atypical cadherins Celsr1-3 differentially regulate migration of facial branchiomotor neurons in mice.

During hindbrain development, facial branchiomotor neurons (FBM neurons) migrate from medial rhombomere (r) 4 to lateral r6. In zebrafish, mutations in planar cell polarity genes celsr2 and frizzled3a block caudal migration of FBM neurons. Here, we investigated the role of cadherins Celsr1-3, and Fzd3 in FBM neuron migration in mice. In Celsr1 mutants (knock-out and Crash alleles), caudal migration was compromised and neurons often migrated rostrally into r2 and r3, as well as laterally. These phenotypes were not caused by defects in hindbrain patterning or neuronal specification. Celsr1 is expressed in FBM neuron precursors and the floor plate, but not in FBM neurons. Consistent with this, conditional inactivation showed that the function of Celsr1 in FBM neuron migration was non-cell autonomous. In Celsr2 mutants, FBM neurons initiated caudal migration but moved prematurely into lateral r4 and r5. This phenotype was enhanced by inactivation of Celsr3 in FBM neurons and mimicked by inactivation of Fzd3. Furthermore, Celsr2 was epistatic to Celsr1. These data indicate that Celsr1-3 differentially regulate FBM neuron migration. Celsr1 helps to specify the direction of FBM neuron migration, whereas Celsr2 and 3 control its ability to migrate.